Semiconductor devices are fabricated through several processes including a process of polishing a dielectric film, e.g., SiO2, and a process of polishing a metal film, e.g., copper or tungsten. Fabrication processes of backside illumination CMOS sensor and through-silicon via (TSV) include a process of polishing a silicon layer (silicon wafer), in addition to the polishing processes of the dielectric film and the metal film.
Polishing of the wafer is terminated when a thickness of a film (e.g., the dielectric film, the metal film, or the silicon layer), constituting a wafer surface, has reached a predetermined target value. CMP (Chemical Mechanical Polishing) apparatus is used for polishing the wafer. FIG. 1 is a schematic view of the CMP apparatus. The CMP apparatus includes a polishing table 101 with a polishing pad 100 attached to an upper surface thereof, a top ring 110 for holding a wafer W, a slurry supply nozzle 115 for supplying a polishing liquid (slurry) onto the polishing pad 100, and a film-thickness measuring device 120 for measuring a film thickness of the wafer W. The film-thickness measuring device 120 is embedded in the polishing table 101.
The top ring 110 and the polishing table 101 rotate as indicated by arrows. In this state, the top ring 110 presses the wafer W against the polishing pad 100, while the slurry supply nozzle 115 supplies the polishing liquid onto the polishing pad 100. The wafer W is polished by sliding contact with the polishing pad 100 in the presence of the polishing liquid. During polishing of the wafer W, the film-thickness measuring device 120 rotates together with the polishing table 101 and measures the film thickness while sweeping a surface of the wafer W as indicated by arrow A. Polishing of the wafer W is terminated when the film thickness has reached the predetermined target value.
An optical film-thickness measuring device is one example of the film-thickness measuring device 120 used in the above-described CMP apparatus. This optical film-thickness measuring device is designed to illuminate the surface of the wafer with light and analyze reflected light from the wafer to determine a thickness of a film formed on the surface of the wafer.
However, the film-thickness measuring device incorporated in the CMP apparatus may fail to measure the thickness if there is a variation in the thickness of the film because it measures the thickness of the film while the device itself is moving as shown in FIG. 1. Moreover, since the film is scraped away every time the polishing table makes one revolution, it is impossible to measure the thickness at the same position under the same conditions.